In the past decades several etiological agents of mammalian disease, in particular of respiratory tract illnesses (RTI), in particular of humans, have been identified. Classical etiological agents of RTI with mammals are respiratory syncytial viruses belonging to the genus Pneumoviruses found with humans (hRSV) and ruminants such as cattle or sheep (bRSV and/or oRSV). In human RSV differences in reciprocal cross-neutralization assays, reactivity of the G proteins in immunological assays and nucleotide sequences of the G gene are used to define 2 hRSV antigenic subgroups. Within the subgroups the aa sequences show 94% (subgroup A) or 98% (subgroup B) identity, while only 53% aa sequence identity is found between the subgroups. Additional variability is observed within subgroups based on monoclonal antibodies, RT-PCR assays and RNAse protection assays. Viruses from both subgroups have a worldwide distribution and may occur during a single season. Infection may occur in presence of pre-existing immunity and the antigenic variation is not strictly required to allow re-infection. See, for example, W. M. Sullender, Respiratory Syncytial Virus Genetic and Antigenic Diversity, Clinical Microbiology Reviews, 2000, 13(1):1-15; P. L. Collins, K. McIntosh, and R. M. Chanock, Respiratory syncytial virus, Fields virology, ed. B. N. Knipe, P. M. Howley, 1996, Philadelphia: Lippencott-Raven, pp. 1313-1351; P. R. Johnson, et al., The G glycoprotein of human respiratory syncytial viruses of subgroups A and B: extensive sequence divergence between antigenically related proteins, Proc. Natl. Acad. Sci. U.S.A., 1987, 84(16):5625-9; P. L. Collins, The molecular Biology of Human Respiratory Syncytial Virus (RSV) of the Genus Pneumovirus, in The Paramyxoviruses, D. W. Kingsbury, Editor, 1991, Plenum Press: New York. p. 103-153.
Another classical Pneumoviruses is the pneumonia virus of mice (PVM), in general only found with laboratory mice. However, a proportion of the illnesses observed among mammals can still not be attributed to known pathogens.